Additive manufacturing or rapid prototyping methods for producing objects comprise layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is deposited on a powder bed in a build chamber and a laser beam is scanned across portions of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required.
It is known to monitor and control the selective laser powder process in response to monitored parameters. For example, WO2007/147221 discloses apparatus for collecting laser light reflected from the powder bed using elements of the optical train that are used to deliver the laser beam. The laser is delivered to the powder bed and radiation reflected by the powder bed is collected by scanning optics comprising a pair of movable mirrors and an f-θ lens. A semi-reflective mirror reflects the laser light towards the scanning optics but allows the reflected radiation to pass therethrough to a camera and photodetector. A similar system is disclosed in WO95/11100 but the beam splitter is provided between the scanning mirrors and a lens for focusing the laser light.
US2007/0176312A1 discloses the use of Raman spectroscopy in direct laser deposition, which allows analysis for organic and/or ceramic constituents. WO2009/112799A2 discloses a device for applying layers of a strong Raman scatterer product and a Raman spectrometer device for monitoring the frequency of light scattered from an area of the material whilst the laser used to melt the material is applied to that area.
It is also known to monitor additive manufacturing processes using IR cameras. For example, US2009/0152771 discloses detecting an IR-radiation image of an applied powder layer and determining defects and/or geometrical irregularities in the applied powder layer form the IR-radiation image.
A problem with such monitoring techniques, particularly when applied to selective laser melting of metals, is that the area being consolidated is obscured from the sensor, such as by plasma generated above a melt pool.